This invention relates to a wear-resistant sintered ferrous alloy for parts subjected to friction and a method of producing the same.
A typical example of metal parts that make continuous rubbing contact with another metal part is the rocker arm of an internal combustion engine. It is usual to form the tip part of the rocker arm by chilled casting separately from the main part of the rocker arm or alternatively to harden the tip portion of the rocker arm made of steel by either a suitable heat treatment such as carbrizing or nitriding or a surface treatment such as chormium plating.
As the performance requirements to the recent internal combustion engines for automotive uses have become more and more severer, there is the tendency to press the rocker arms against the cams under increased pressures. Then there arises a problem that the supply of lubricating oil into the interface between the cam surface and the rocker arm tips becomes insufficient while the cam rotation rate is low and hence the sliding speed of the rocker arm tip relative to the cam surface is low as occurs during idling or very low speed operation of the engine. The insufficiency in lubrication often results in serious wear or scuffing of the rocker arm tip made of a usual material or peeling of the plated hard film from the rocker arm body.
It is possible to use a sintered porous alloy that is impregnated with oil to become self-lubricating and resistant to wear. In that case, however, the sintered alloy needs to contain relatively large amounts of special and costly metals such as W and/or Mo in order to acquire hardness sufficient for use in parts subjected to severe rubbing or friction. If the contents of such costly metals are decreased to reduce the cost of the alloy, then it becomes necessary to subject the sintered alloy parts to a hardening treatment such as heat treatment or surface treatment so that the production of the alloy parts becomes complicate with inevitable rise in the production cost.